The present invention relates to the reception of binary data sent by an integrated circuit with contactless operation. An integrated circuit with contactless operation may include a contactless chip card, an electronic label, and an electronic token, for example.
FIG. 1 is a standard circuit diagram of a detection system 10 and a load modulation system 20 cooperating with each other by inductive coupling. The system 10 includes an adjustable capacitor C1 and a coil L1 connected in series to form a resonant circuit. The capacitor C1 is connected by a resistor R1 to the output of a voltage/current amplifier 1. The voltage/current amplifier 1 receives at an input an AC voltage V0 having a frequency F0 generated by an oscillator 2. The voltage V0, converted into current by the amplifier 1, forms the excitation signal for the resonant circuit L1C1 having a natural frequency Fp1 set near the excitation frequency F0.
The load modulation system 20 includes an antenna coil L2 forming a resonant circuit with a capacitor C2 having a natural frequency Fp2 tuned to the frequency F0. The coil L2 is parallel-connected with a load modulation circuit, which in this case, is a resistor R2 series connected with a switch Tm. The switch Tm is controlled by a binary signal S1 having a carrier frequency F1 that is to be detected by the detection system 10. The load modulations applied to the coil L2 as a function of the signal S1 effect the coil L1 by inductive coupling. An AC voltage Vm having a frequency F0 and modulated in amplitude and in phase is observed at the terminals of the antenna circuit L1C1.
According to the method illustrated in FIG. 1, detection of the load modulation signal S1 is done by a phase comparator 3 receiving the voltages V0 and Vm. The comparator 3 delivers a voltage Vj proportional to the phase shift of these two signals. The voltage Vj is filtered by a bandpass filter 4 set to the carrier frequency F1 of the signal S1. The output of the bandpass filter 4 is made binary by a comparator 5 with a threshold Vref. At the output of the comparator 5 is the signal S1 having a frequency F1.
Detecting the signal S1 by phase comparison has the advantage of providing a good signal-to-noise ratio, but requires the natural frequency Fp1 of the antenna circuit L1C1 to be substantially mismatched with respect to the frequency F0. The phase j of the voltage Vm is not sensitive to the load modulations when the natural frequency Fp1 of the antenna circuit L1C1 is exactly equal to the excitation frequency F0.
FIG. 2 shows the phase curves j of the voltage Vm as a function of the natural frequency Fp1 for various resistance values of 0, v1, v2 for the load resistor R2. When Fp1 is equal to F0, the phase curves intersect at the same point and the sensitivity of the system 10 to the load modulations is zero. In setting the natural frequency Fp1 of the circuit L1C1 to a point F0' close to F0, high sensitivity is obtained. The phase j is a function of the load or load resistor R2.
The natural frequency Fp1 of the circuit L1C1 must be precisely adjusted by setting the capacitor C1, and must be regularly checked to correct any drifts of the capacitor caused by environmental conditions (temperature, humidity, etc.) and aging. This constraint makes the method undesirable for implementation in a system with limited maintenance. For example, a system of limited maintenance with respect to contactless chip cards is a contactless chip card reader located in a public place, or a station for automatic testing of contactless integrated circuits, etc. For this reason, it is generally preferred to use a second method which includes demodulation of the amplitude of the voltage Vm at the terminals of the resonant circuit L1C1 to extract the signal S1 therefrom. This second method, however, has the drawback of providing a poor signal-to-noise ratio.